The apparatus described herein relates generally to a drive train for a wind turbine. More specifically, the apparatus relates to an integrated medium-speed geared drive train for a wind turbine having a non-rotating ring gear.
Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. Wind turbines do not emit greenhouse gases (GHGs), and therefore, do not contribute to global warming. With the growing interest in wind generated electricity, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Wind is usually considered to be a form of solar energy caused by uneven heating of the atmosphere by the sun, irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetation. The terms wind energy or wind power, describe the process by which the wind is used to rotate a shaft and subsequently generate mechanical power or electricity.
Typically, wind turbines are used to convert the kinetic energy in the wind into mechanical power. This mechanical power may be used for specific tasks (such as grinding grain or pumping water) or a generator may convert this mechanical power (i.e., the rotation of a shaft) into electricity. A wind turbine usually includes an aerodynamic mechanism (e.g., blades) for converting the movement of air into a mechanical motion (e.g., rotation), which is then converted with a generator into electrical power. Power output from the generator is proportional to the cube of the wind speed. As wind speed doubles, the capacity of wind generators increases almost eightfold.
The majority of commercially available wind turbines utilize geared drive trains to connect the turbine blades to the electrical generators. The wind turns the turbine blades, which spin a low speed shaft, which feeds into a gearbox having a higher speed output shaft. This higher speed output shaft connects to a generator and makes electricity. The geared drive aims to increase the velocity of the mechanical motion.
The majority of geared drive trains in existing wind turbines of ratings >1 MW utilize 3 gear stages to achieve gear ratios ranging from about 1:70 up to about 1:110. The three stages typically comprise a simple planetary or epicylic first stage, followed by two parallel offset stages (bull-gear+pinion gears) or a second simple planetary stage followed by a one parallel offset stage. The high gear ratio enables a generator that is substantially smaller and lower cost than the gearbox. The relatively high-speed of the generator forces the generator to have an aspect ratio that is longer than it is wide, with radial-vented cooling. The high-speed output shaft of the gearbox is generally not concentric with the low-speed input shaft of the gearbox. For these reasons, the generator is mounted separate from the gearbox, thereby requiring additional couplings, support frames, and generator frame mass.
With the advent of cost-effective high-efficiency permanent magnet (PM) synchronous generators, the combination of a 3-stage gearbox and separately mounted high-speed generator is no longer the optimal configuration in terms of system mass, size, cost, and efficiency. For example, gearboxes, unlike most generators, are typically a major source of unreliability and unavailability in current wind turbines. Furthermore, the high-speed gear stage often creates unacceptable acoustic noise that radiates from the wind turbine.
A more optimal configuration of a geared drive train is therefore strongly desired by the wind industry to provide increased reliability and availability, reduced cost, reduced mass and size, and increased efficiency.